Health Effects: May cause irritation of the eyes, nose, and throat. Many chemicals can cause serious problems in high concentrations but are present in the home at very low concentrations. The health impact of mixtures of these chemicals is not known.

Health Effects: CO itself is an odorless gas that can kill. Lower concentrations cause headaches, weakness, and drowsiness. Symptoms are often mistaken for the flu.

Sources: This chemical is found in many products because of several very useful properties. Drapes and permanent press fabrics can contain formaldehyde. Interior wood products (particle board, paneling, fiberboard) used to make countertops, cabinets and veneered furniture introduce large amounts of formaldehyde into the home.

Health Effects: Formaldehyde is a strong irritant. About 10 percent of the population is sensitive to extremely low levels.

Sources: Dust mites thrive in mattresses because of warmth, moisture, and a good supply of food. They may also live in carpets.

Health Effects: Many people are allergic to dust. Dust mites—microscopic spider-like creatures—are the primary source of allergens and a leading cause of asthma. Asthma-related emergency room admissions and deaths have increased in recent years (deaths tripled in ten years from 1,674 in 1977 to 4,580 in 1988).

Sources: These organisms can grow almost anywhere, and their spores are nearly everywhere. Moisture is an essential ingredient for the growth of mold, mildew, etc. Organisms will also grow in stagnant water. Growth of these microorganisms is mainly a nuisance, in terms of unpleasant odors and destruction of materials. Sometimes problems can be more serious, involving health problems and structural damage to buildings (rotting wood).

Health Effects: These usually become important when molds or by-products are dispersed so that they are inhaled by people (mold growth in duct work, drip trays under air conditioning units or refrigerators). Some humidifiers can disperse large amounts of mold growth via droplets of water.

Sources: Asbestos was used in many products, ranging from insulating pipe and duct wrap to vinyl floor tiles and spackling. Thus, nearly all homes more than about 20 years old are likely to have some asbestos. If the material is in good condition and is left undisturbed, asbestos fibers should not become airborne and will not become a health hazard.

Health Effects: Asbestos can cause lung cancer, especially among smokers. About 600 to 1,000 U.S. deaths each year are attributed to asbestos; nearly all involve workplace exposure.

Sources: Radon is a colorless, odorless, radioactive gas that comes from uranium. Houses on or near sources can draw in radon from soil. Ground wells, natural gas, and stone used for construction are other potential sources of radon.

Health Effects: Of all indoor pollutants, radon is one of the most important in terms of numbers of deaths attributed to it. Radon can cause lung cancer with prolonged exposure. Current calculations estimate 5,000 to 15,000 deaths each year in the U.S. are caused by radon.

Sources: Direct exposure from smoking tobacco. Indirect exposure from secondhand smoke. Health risks from tobacco smoke are far greater than from any other indoor air pollutant.

Health Effects: Tobacco smoke has been officially designated as a known human carcinogen. It contains many indoor air pollutants (carbon monoxide, formaldehyde, VOCs, particulates, etc.). The illnesses and deaths attributed to cigarette smoking are documented by very strong medical evidence and make most other health risks look trivial by comparison. Some 434,000 deaths in the U.S. each year are attributed to tobacco smoking.

Environmental tobacco smoke is a problem for nonsmokers who are regularly exposed to tobacco smoke. Some 3,000 lung cancer deaths each year are attributed to secondhand smoke. Children are particularly susceptible to cigarette smoke. It increases the severity of asthma and is estimated to cause between 150,000 and 300,000 cases of bronchitis and pneumonia in young children each year.